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Showing papers by "Ames Research Center published in 1987"


Journal ArticleDOI
TL;DR: In this article, a direct numerical simulation of a turbulent channel flow is performed, where the unsteady Navier-Stokes equations are solved numerically at a Reynolds number of 3300, based on the mean centerline velocity and channel half-width, with about 4 million grid points.
Abstract: A direct numerical simulation of a turbulent channel flow is performed. The unsteady Navier-Stokes equations are solved numerically at a Reynolds number of 3300, based on the mean centerline velocity and channel half-width, with about 4 million grid points. All essential turbulence scales are resolved on the computational grid and no subgrid model is used. A large number of turbulence statistics are computed and compared with the existing experimental data at comparable Reynolds numbers. Agreements as well as discrepancies are discussed in detail. Particular attention is given to the behavior of turbulence correlations near the wall. A number of statistical correlations which are complementary to the existing experimental data are reported for the first time.

4,788 citations


Journal ArticleDOI
06 Nov 1987-Science
TL;DR: The exotic nitrogen-fixing tree Myrica faya invades young volcanic sites where the growth of native plants is limited by a lack of nitrogen, thereby altering the nature of ecosystem development after volcanic eruptions.
Abstract: The exotic nitrogen-fixing tree Myrica faya invades young volcanic sites where the growth of native plants is limited by a lack of nitrogen. Myrica quadruples the amount of nitrogen entering certain sites and increases the overall biological availability of nitrogen, thereby altering the nature of ecosystem development after volcanic eruptions.

890 citations


Journal ArticleDOI
Chul Park1
TL;DR: In this paper, a two-temperatur e chemical-kinet ic model for air is assessed by comparing theoretical results with existing experimental data obtained in shock tubes, ballistic ranges, and flight experiments.
Abstract: A two-temperatur e chemical-kinet ic model for air is assessed by comparing theoretical results with existing experimental data obtained in shock tubes, ballistic ranges, and flight experiments. In the model, one temperature (T) is assumed to characterize the heavy-particle translational and molecular rotational energies, and another temperature (Tv) the molecular vibrational, electron translational, and electronic excitation energies. The theoretical results for nonequilibrium flow in shock tubes are obtained using the computer code STRAP (shock-tube radiation program) and for flow along the stagnation streamline in the shock layer over spherical bodies using the newly developed code SPRAP (stagnation-point radiation program). Substantial agreement is shown between the theoretical and experimental results for relaxation times and radiative heat fluxes. At very high temperatures, the spectral calculations need further improvement. The present agreement provides strong evidence that the two-temperature model characterizes principal features of nonequilibriu m airflow. New theoretical results using the model are presented for the radiative heat fluxes at the stagnation point of 6 m radius sphere, representing an aeroassisted orbital transfer vehicle, over a range of freestream conditions. Assumptions, approximations, and limitations of the model are discussed. Nomenclature = average molecular speed ^/$kT/nm, cm s ~ ! = pre-exponential factor in reaction rate coefficient, cm3mole~ 1 s~ * - average vibrational energy per particle, erg = average vibrational energy per particle under equilibrium, erg = reaction energy, erg

690 citations


Journal ArticleDOI
01 Aug 1987-Icarus
TL;DR: Theoretical arguments are presented in support of the idea that Mars possessed a dense CO2 atmosphere and a wet, warm climate early in its history and the plausibility of such a CO2 greenhouse is tested by formulating a simple model of the CO2 geochemical cycle on early Mars.

568 citations


Journal ArticleDOI
TL;DR: Climate models suggest that carbon dioxide concentrations during the Archean must have been at least 100-1000 times the present level to keep the Earth's surface temperature above freezing in the face of decreased solar luminosity, but periods of extensive glaciation during the early and late Proterozoic indicate that the climate at these times was relatively cool.

474 citations


Journal ArticleDOI
TL;DR: A transform which maps an image into a set of images that vary in resolution and orientation, each pixel in the output may be regarded as the simulated response of a neuron in human visual cortex is described.
Abstract: With a goal of providing means for accelerating the image processing, machine vision, and testing of human vision models, an image transform was designed, which makes it possible to map an image into a set of images that vary in resolution and orientation. Each pixel in the output may be regarded as the simulated response of a neuron in human visual cortex. The transform is amenable to a number of shortcuts that greatly reduce the amount of computation.

466 citations


Journal ArticleDOI
TL;DR: In this article, a one-parameter family of explicit and implicit total variation diminishing (TVD) schemes is developed which permits incorporation of an expanded group of slope and flux limiters.

448 citations


Journal ArticleDOI
TL;DR: In this article, the structures of the vorticity fields in several homogeneous irrotational straining flows and a homogeneous turbulent shear flow were examined using a database generated by direct numerical simulation of the unsteady Navier-Stokes equations.
Abstract: The structures of the vorticity fields in several homogeneous irrotational straining flows and a homogeneous turbulent shear flow were examined using a database generated by direct numerical simulation of the unsteady Navier-Stokes equations. In all cases, strong evidence was found for the presence of coherent vortical structures. The initially isotropic vorticity fields were rapidly affected by imposed mean strain and the rotational component of mean shear and developed accordingly. In the homogeneous turbulent shear-flow cases, the roll-up of mean vorticity into characteristic hairpin vortices was clearly observed, supporting the view that hairpin vortices are an important vortical structure in all turbulent shear flows; the absence of mean shear in the homogeneous irrotational straining flows precludes the presence of hairpin-like vortices.

348 citations


Journal ArticleDOI
TL;DR: In this paper, the authors measured velocities in a reattaching turbulent shear layer behind a rearward facing step in an effort to understand unsteady behaviour of reattached flows.
Abstract: Conditionally sampled velocities have been measured in a reattaching turbulent shear layer behind a rearward facing step in an effort to understand unsteady behaviour of reattaching flows. Laser-Doppler velocimeter measurements were conditionally sampled on the basis of instantaneous flow direction near reattachment. Conditions of abnormally short reattachment and abnormally long reattachment were considered. Ensemble-averages of measurements made during these conditions were used to obtain mean velocities and Reynolds stresses. In the mean flow, conditional streamlines show a global change in flow pattern which correlates with wall-flow direction. This motion can loosely be described as a "flapping" of the shear layer. Stresses shown also vary with the change in flow pattern. Yet, the global "flapping" motion does not appear to contribute much to the fluctuating energy in the flow. A second type of fluctuating motion (vortical) was observed. Spectral analysis of both wall static pressure and streamwise velocity show that the majority of energy in the flow resides in frequencies characteristic of roll-up and pairing of vortical structure seen in free shear layers (St = 0.2). Two-point velocity correlations also indicate a vortical behaviour of the flow. It is conjectured that the "flapping" is a disorder of the roll-up and pairing process occurring in the shear layer.

293 citations


Journal ArticleDOI
TL;DR: In this paper, the authors analyzed the relationship between remote sensing data and the leaf area index (LAI) of coniferous forests using data acquired by an Airborne Thematic Mapper.

278 citations


Journal ArticleDOI
TL;DR: In this paper, an attempt is made to analyze two-dimensional flow past the rotor/stator con- figuration of an axial turbine using state-of-the-art tools and computing facilities.
Abstract: An accurate numerical analysis of the flows associated with rotor/stator configurations in turbomachinery can be very helpful in optimizing performance. In this study, the unsteady, thin-layer, Navier-Stokes equations are solved using a system of patched and overlaid grids for a rotor/stator configuration of an axial turbine. The equations necessary for an accurate transfer of information between the several grids are briefly described within the framework of an iterative, implicit algorithm. Results in the form of Mach number contours, time-averaged pressures, unsteady pressures, amplitudes, and phase are presented. The numerical results are also compared with experimental data and the agreement is found to be good. HE aerodynamic processes associated with the flow of fluid through turbomachines pose one of the toughest challenges to the computational fluid dynamicist. The un- steady nature of the flow, the complex geometries involved, the motion of some parts of the system relative to others, and the periodic transition of the flow from laminar to tur- bulent are some of the factors that contribute to the com- plexity of the problem. A clear understanding of these types of flows is essential for the optimization of the performance of turbomachiner y. In this study, an attempt is made to analyze two-dimensional flow past the rotor/stator con- figuration of an axial turbine using state-of-the-art computa- tional tools and computing facilities. The two-dimensional analysis of stator airfoils in isolation or rotor airfoils in isolation is a relatively straightforward task. Such an analysis is valid when the two rows of blades are set far enough apart so that the interaction effects are minimal. However, the desire to minimize engine length re- quires the stator and rotor airfoils to be closely spaced. Clearly, the interaction effects will become more important as the axial gap between the rows is reduced. In fact, the flow becomes periodically unsteady for small values of the axial gap. The experimental results of Ref. 1 show that the temporal pressure fluctuation (the difference between the minimum and maximum pressure values) near the leading edge of the rotor can be as much as 12% of the exit dynamic pressure when the axial gap is reduced to 15% of the chord length (for the operating conditions and geometry chosen). Hence, it is important that the rotor and stator airfoils be treated as a single system when the interaction effects become predominant. A computational tool that provides the design engineer with the necessary aerodynamic data can be used to great advantage in redesigning rotor and stator airfoils to enhance performance. Such a tool has to accurately simulate the unsteady flow associated with rotor/stator con- figurations exhibiting a strong interaction. A finite-differe nce solution to the Navier-Stokes equations requires the generation of a computational grid for the

Proceedings ArticleDOI
01 Jan 1987
TL;DR: In this article, a head-mounted, wide-angle, stereoscopic display system controlled by operator position, voice and gesture has been developed for use as a multipurpose interface environment.
Abstract: A head-mounted, wide-angle, stereoscopic display system controlled by operator position, voice and gesture has been developed for use as a multipurpose interface environment. The system provides a multisensory, interactive display environment in which a user can virtually explore a 360-degree synthesized or remotely sensed environment and can viscerally interact with its components. Primary applications of the system are in telerobotics, management of large-scale integrated information systems, and human factors research. System configuration, application scenarios, and research directions are described.

Journal ArticleDOI
TL;DR: A code modeled on the simple cells of the primate striate cortex is explored, which maps a digital image into a set of subimages (layers) that are bandpass in spatial frequency and orientation and which is reconstructed from the code.
Abstract: Hypothetical schemes for neural representation of visual information can be expressed as explicit image codes. Here, a code modeled on the simple cells of the primate striate cortex is explored. The Cortex transform maps a digital image into a set of subimages (layers) that are bandpass in spatial frequency and orientation. The layers are sampled so as to minimize the number of samples and still avoid aliasing. Samples are quantized in a manner that exploits the bandpass contrast-masking properties of human vision. The entropy of the samples is computed to provide a lower bound on the code size. Finally, the image is reconstructed from the code. Psychophysical methods are derived for comparing the original and reconstructed images to evaluate the sufficiency of the code. When each resolution is coded at the threshold for detection artifacts, the image-code size is about 1 bit/pixel.

Journal ArticleDOI
13 Nov 1987-Science
TL;DR: Focused studies of the interactions between the atmosphere and the biosphere that regulate trace gases can improve both the understanding of terrestrial ecosystems and the ability to predict regional- and global-scale canges in atmospheric chemistry.
Abstract: Many biogenic trace gases are increasing in concentration or flux or both in the atmosphere as a consequence of human activities. Most of these gases have demonstrated or potential effects on atmospheric chemistry, climate, and the functioning of terrestrial ecosystems. Focused studies of the interactions between the atmosphere and the biosphere that regulate trace gases can improve both our understanding of terrestrial ecosystems and our ability to predict regional-and global-scale canges in atmospheric chemistry.

Journal ArticleDOI
TL;DR: In this article, a simple matrix expression is obtained for the strain components of a beam in which the displacements and rotations are large, and the only restrictions are on the magnitudes of the strain and of the local rotation, a newly-identified kinematical quantity.
Abstract: A simple matrix expression is obtained for the strain components of a beam in which the displacements and rotations are large. The only restrictions are on the magnitudes of the strain and of the local rotation, a newly-identified kinematical quantity. The local rotation is defined as the change of orientation of material elements relative to the change of orientation of the beam reference triad. The vectors and tensors in the theory are resolved along orthogonal triads of base vectors centered along the undeformed and deformed beam reference axes, so Cartesian tensor notation is used. Although a curvilinear coordinate system is natural to the beam problem, the complications usually associated with its use are circumvented. Local rotations appear explicitly in the resulting strain expressions, facilitating the treatment of beams with both open and closed cross sections in applications of the theory. The theory is used to obtain the kinematical relations for coupled bending, torsion extension, shear deformation, and warping of an initially curved and twisted beam.

Journal ArticleDOI
TL;DR: In this paper, the effects of streamline curvature by comparing the concave and convex sides of the channel were investigated and it was found that stationary Taylor-Gortler vortices were present and that they had a significant effect on the flow by contributing to the mean Reynolds shear stress, enhancing the asymmetry and affecting the underlying turbulence.
Abstract: Low-Reynolds-number, mildly curved, turbulent channel flow has been simulated by direct numerical solution of the Navier – Stokes equations. Computed velocity fields were found to be in good agreement with experimental measurements. The resulting flow fields were used to study the effects of streamline curvature by comparing the concave and convex sides of the channel. Observed effects are consistent with experimental measurements for mild curvature. The most significant difference in the turbulence statistics is in the Reynolds shear stress. This is accompanied by significant differences in the terms of the equation for Reynolds-shear-stress budget. In addition, it was found that stationary Taylor – Gortler vortices were present and that they had a significant effect on the flow by contributing to the mean Reynolds shear stress, enhancing the asymmetry of the channel, and affecting the underlying turbulence.

Journal ArticleDOI
TL;DR: The advantages and difficulties of using various composite grid schemes are reviewed and a trend in computational aerodynamics has been toward the use of composite grids.
Abstract: In finite difference flow field simulations the use of a single well-ordered body-conforming curvilinear mesh can lead to efficient solution procedures. However, it is generally impractical to build a single grid of this type for complex three-dimensional aircraft configurations. As a result, a trend in computational aerodynamics has been toward the use of composite grids. Composite grids use more than one grid to mesh an overall configuration with each individual subgrid of the system patched or overset together. Because each individual subgrid in the system is well ordered, the overall grid is suitable for efficient finite difference solution using vectorized or multitasking computers. Some of the advantages and difficulties of using various composite grid schemes are reviewed in this paper.

Journal ArticleDOI
01 Jan 1987
TL;DR: In this article, the authors provide an overview of the design decisions that must be made so as to provide a context for the decisions made in a number of existing and developing packet radio networks.
Abstract: There are many design choices that must be made in the development of a packet radio network. There is usually no single correct choice, and the decisions are dependent on the environment that the network must work in, the requirements for performance and other functionalities, and the cost and other limitations, in addition, as new hardware and software technologies become available, the parameters governing the decisions change and often result in different selections. This paper outlines a number of design issues and choices available. The intent is to provide an overview of the design decisions that must be made so as to provide a context for the decisions made in a number of existing and developing packet radio networks. It is hoped that this will allow future designs to take advantage of both the wealth of experience available as well as new technologies. Three areas of design decisions are identified. The first area deals with the physical aspects of the network and concentrates on the radio connectivity and channel sharing. The second area deals with the automated management of the network and concentrates on issues such as link management and routing. The third area deals with the interface of the network to the users and some practical aspects of operating and maintaining a network.

Journal ArticleDOI
TL;DR: In this article, the location of the rapid rotators in color-magnitude diagrams and the correlation between chromospheric activity and rotation were examined for the Pleiades K and M dwarfs.
Abstract: The available spectral type and color data for late-type Pleiades members have been reanalyzed, and new reddening estimates are obtained. New photometry for a small number of stars and a compilation of H-alpha equivalent widths for Pleiades dwarfs are presented. These data are used to examine the location of the rapid rotators in color-magnitude diagrams and the correlation between chromospheric activity and rotation. It is shown that the wide range of angular momenta exhibited by Pleiades K and M dwarfs is not necessarily produced by a combination of main-sequence spin-downs and a large age spread; it can also result from a plausible spread in initial angular momenta, coupled with initial main-sequence spin-down rates that are only weakly dependent on rotation. The new reddening estimates confirm Breger's (1985) finding of large extinctions confined to a small region in the southern portion of the Merope nebula.

Book ChapterDOI
01 Jan 1987
TL;DR: In this paper, the physical principles of grain surface chemistry are applied to the formation of molecular hydrogen and icy grain mantles inside dense molecular clouds and the astrophysical consequences are briefly examined.
Abstract: The observational constraints on the composition of the interstellar dust are analyzed. The dust in the diffuse interstellar medium consists of a mixture of stardust (amorphous silicates, amorphous carbon, polycyclic aromatic hydrocarbons, and graphite) and interstellar medium dust (organic refractory material). Stardust seems to dominate in the local diffuse interstellar medium. Inside molecular clouds, however, icy grain mantles are also important. The structural differences between crystalline and amorphous materials, which lead to differences in the optical properties, are discussed. The astrophysical consequences are briefly examined. The physical principles of grain surface chemistry are discussed and applied to the formation of molecular hydrogen and icy grain mantles inside dense molecular clouds. Transformation of these icy grain mantles into the organic refractory dust component observed in the diffuse interstellar medium requires ultraviolet sources inside molecular clouds as well as radical diffusion promoted by transient heating of the mantle. The latter process also returns a considerable fraction of the molecules in the grain mantle to the gas phase.

Journal ArticleDOI
TL;DR: In this article, soil nitrogen transformations and nitrous oxide flux across the soil-air interface were measured in a range of tropical forest sites in Costa Rica, Brazil, and Hawaii.
Abstract: Soil nitrogen transformations and nitrous oxide flux across the soil-air interface were measured in a range of tropical forest sites in Costa Rica, Brazil, and Hawaii. Nitrogen mineralization and nitrification potentials were high in the relatively fertile Costa Rica sites and the Amazonian oxisol/ultisols (>1.5 μg g−1 d−1 of N mineralized), intermediate in Amazonian white sand soils (0.5-0.8 μg g−1 d−1) and low in the Hawaiian montane sites (<0.5 μg g−1 d−1). Nitrous oxide fluxes ranged from 0 to 6.2 ng cm−2 h−1 of N; mean flux per site was highly correlated with mean nitrogen mineralization across 26 sites. These correlated patterns of nitrogen cycling and trace gas production could be useful in the development of regional- and global-scale estimates of nitrous oxide fluxes from tropical forests.

Journal ArticleDOI
01 Jun 1987-Ecology
TL;DR: In this paper, nitrogen transformations and loss following forest clearing in a relatively fertile tropical forest site were measured, showing that nitrogen mineralization, nitrification and amounts of ammonium and nitrate increased substantially in surface soils during the 6 mo following burning, then returned to background levels.
Abstract: We measured nitrogen transformations and loss following forest clearing in a relatively fertile tropical forest site. Nitrogen mineralization, nitrification, and amounts of ammonium and nitrate increased substantially in surface soils during the 6 mo following burning, then returned to background levels. The nitrogen content of microbial biomass declined to half its original value 6 mo after clearing and remained low in the cleared sites. Plant uptake of nitrogen was substantial on cleared plots (50 g/m2), but it accounted for only 18% of 15N label added to field plots. Microbial immobilization of 15N was small relative to that in a cleared temperate site, and measurements of denitrification potentials suggested that relatively little mineralized nitrogen was lost to the atmosphere. Substantial amounts of nitrogen (40-70 g/m2) were retained as exchangeably bound nitrate deep in the soils of a cleared plot on which revegetation was prevented; this process accounted for 12% of the 15N label added to field plots.

Journal ArticleDOI
01 Jun 1987-Icarus
TL;DR: In this article, four hypotheses concerning the origin of the layered deposits in the Martian Valles Marineris, whose individual thicknesses range from about 70 to 300 m, were discussed, including: aeolian deposition, deposition of remnants of the material constituting the canyon walls, and deposition in standing bodies of water.

Journal ArticleDOI
03 Jul 1987-Science
TL;DR: Observations suggest that some of the carbonaceous material in IDPs and meteorites may have been produced in circumstellar dust shells and only slightly modified in interstellar space.
Abstract: Raman spectra of interplanetary dust particles (IDPs) and meteorites containing material similar to polycyclic aromatic hydrocarbons (PAHs) show features that are similar in position and relative strength to interstellar IR emission features attributable to vibrational transitions in free molecular-sized PAHs. In addition, these spectra sometimes show red photoluminescence that has elsewhere been attributed to PAHs, and a part of the carbonaceous phase in IDPs and meteorites contain a degree of deuterium enrichment anticipated in small, free PAHs that are exposed to ISM UV radiation. These observations suggest that some of the IDPs' carbonaceous material may have been produced in circumstellar dust shells, and only slightly modified in interstellar space.

Journal ArticleDOI
TL;DR: In this article, isotopic composition of C, H, and N in insoluble organic fraction from the Murchison CM chondrite were used to discriminate between different theories for the origin of prebiotic organic material in the early solar system.

Journal ArticleDOI
TL;DR: In this paper, energy optimized near Hartree-Fock quality Gaussian basis sets are presented for the ground states of the second-row atoms for Na(2P), Na(+), Na(-), Mg(3P), P(-), S(-), and Cl(-).
Abstract: Energy optimized, near Hartree-Fock quality Gaussian basis sets ranging in size from (17s12p) to (20s15p) are presented for the ground states of the second-row atoms for Na(2P), Na(+), Na(-), Mg(3P), P(-), S(-), and Cl(-). In addition, optimized supplementary functions are given for the ground state basis sets to describe the negative ions, and the excited Na(2P) and Mg(3P) atomic states. The ratios of successive orbital exponents describing the inner part of the 1s and 2p orbitals are found to be nearly independent of both nuclear charge and basis set size. This provides a method of obtaining good starting estimates for other basis set optimizations.

Journal ArticleDOI
TL;DR: The FDDI Token Ring Protocol (FDDI) as discussed by the authors is a protocol that uses a timed token protocol, in which noncritical messages may be transmitted only if recent movement of the token among stations has been sufficiently fast relative to a target token rotation time (TTRT).
Abstract: The FDDI Token Ring Protocol controls communication over fiber optic rings with transmission rates in the range of 100 megabits per second. It is intended to give guaranteed response to time-critical messages by using a "timed token" protocol, in which non-critical messages may be transmitted only if recent movement of the token among stations has been sufficiently fast relative to a "target" token rotation time (TTRT).

Journal ArticleDOI
M. Johnson1
TL;DR: This paper proves that FDDI timing requirements are satisfied, i.e., the token rotates quickly enough to prevent initiation of recovery unless there is failure of a physical resource or unless the network management entity within a station initiates the recovery process.
Abstract: The fiber distributed data interface (FDDI) is an ANSI draft proposed standard for a 100 Mbit/s fiber-optic token ring. The FDDI timed token access protocol provides dynamic adjustment of the load offered to the ring, with the goal of maintaining a specified token rotation time and of providing a guaranteed upper bound on time between successive arrivals of the token at a station. FDDI also provides automatic recovery when errors occur. The bound on time between successive token arrivals is guaranteed only if the token rotates quickly enough to satisfy timer requirements in each station when all ring resources are functioning properly. Otherwise, recovery would be initiated unnecessarily. The purpose of this paper is to prove that FDDI timing requirements are satisfied, i.e., the token rotates quickly enough to prevent initiation of recovery unless there is failure of a physical resource or unless the network management entity within a station initiates the recovery process.

Journal ArticleDOI
TL;DR: In this article, the authors examined perspective projections of cockpit displays of traffic information as a means of presenting aircraft separation information to airline pilots and found that pilots took more time to select avoidance maneuvers with a conventional plan-view display than with an experimental perspective display.
Abstract: Part-task experiments have examined perspective projections of cockpit displays of traffic information as a means of presenting aircraft separation information to airline pilots. Ten airline pilots served as subjects in an experiment comparing the perspective projection with plan-view projections of the same air traffic situations. The pilots' task was to monitor the traffic display in order to decide if an avoidance maneuver was needed. Pilots took more time to select avoidance maneuvers with a conventional plan-view display than with an experimental perspective display. In contrast to previous results, if the pilots selected a maneuver with the perspective display, they were more likely to choose one with a vertical component. Tabulation of the outcomes of their initial avoidance decisions with both perspective and plan-view displays showed that they were more likely to achieve required separation with maneuvers chosen with the aid of perspective displays.

Journal ArticleDOI
Gevarter1
TL;DR: In this article, the authors review the factors that constitute an Expert System Building Tool (ESBT) and evaluate current tools in terms of these factors, based on their structure and their alternative forms of knowledge representation, inference mechanisms and developer/end-user interfaces.
Abstract: This memorandum reviews the factors that constitute an Expert System Building Tool (ESBT) and evaluates current tools in terms of these factors. Evaluation of these tools is based on their structure and their alternative forms of knowledge representation, inference mechanisms and developer/end-user interfaces. Next, functional capabilities, such as diagnosis and design, are related to alternative forms of mechanization. The characteristics and capabilities of existing commercial tools are then reviewed in terms of these criteria.